Recently, a display device such as a large display or a projector has greatly increased in performance. In such a display device, polarizing directions must be aligned in many cases. For example, in a liquid crystal display device widely used for a flat display, polarizers are arranged in both the incident side and the exit side of light from a light source and, by applying a voltage to a liquid crystal sandwiched between the polarizers, the polarized rotation of the incident light is controlled, and transmittance of the light to the exit side is controlled. The polarizer used for aligning the polarized light has characteristics of transmitting liner polarized light of a predetermined direction and reflecting or absorbing linear polarized light orthogonal to the predetermined direction.
Among the polarizers, a reflective polarizer, which is a type for reflecting one polarized light, can reuse the reflected light by converting the polarizing direction of the reflected light by a wave plate or the like. Thus, the reflective polarizer can achieve high light use efficiency. As a result, power can be saved for the display device.
As the reflective polarizer, a wire grid polarizer having a grid structure (wire grid structure) where many very small thin metal wires are arrayed is widely used. The wire grid structure, in which, for example, a plurality of thin metal wires are arranged in parallel at repeated intervals (pitches) of a hundred and several tens of nm with widths of several tens of nm and heights of hundred nm, is described in detail in, for example, Patent Literature 1.
In the wire grid polarizer, light polarized in a direction orthogonal to the longitudinal direction of the thin metal wire, namely, the extending direction of the thin metal wire, (hereinafter, TM light) is transmitted, while light polarized in a direction parallel to the longitudinal direction of the thin metal wire (hereinafter, TE light) is reflected. As a metallic material for the thin metal wire, aluminum is generally used.
An example of a light-emitting device using such a reflective polarizer is a polarized light emitting diode (polarized LED). This polarized LED is described in, for example, Patent Literature 1. Light emitted from the LED as a light source is not aligned in polarization. Thus, when the light is directly emitted from the LED to the liquid crystal plate, only a half of the emitted light can be used. The polarizing direction of the emitted light can be aligned in one direction by using a polarization conversion element. However, when the polarization conversion element is used, the optical path of the polarized and converted light is added. This doubles the substantial light-emitting area. In such a case, since Etendue as a product of the light-emitting area and a radial solid angle increases, light use efficiency is reduced in the case of use in the projector or the like.
Under these circumstances, there has been offered a structure where the reflective polarizer is disposed on the emitting surface of the LED and one polarized light reflected by the reflective polarizer is returned to the LED side to be reused. This structure can achieve high light use efficiency because polarized light can be aligned without increasing the Etendue. In this structure, since the polarizer is located directly above the LED, the polarizer is exposed to high light density, and the temperature of the polarizer increases. The wire grid structure is highly reliable because it can be made of only an inorganic material, and its life can be long even in an environment where the temperature of the polarizer increases.